145
Canadian Metallurgical Quarterly, Vol 45, No 2 pp 145-152, 2006
© Canadian Institute of Mining, Metallurgy and Petroleum
Published by Canadian Institute of Mining, Metallurgy and Petroleum
Printed in Canada. All rights reserved
RECOVERY OF METALS FROM COPPER CONVERTER SLAG
BY LEACHING WITH K2Cr2O7-H2SO4
M. BOYRAZLI1, H.S. ALTUNDOĞAN2 and F. TÜMEN2
1
Department of Metallurgical and Materials Engineering, Fırat University, 23279-Elazığ, Turkey
2
Department of Chemical Engineering, Fırat University, 23279-Elazığ, Turkey
(Received in revised form October, 2005)
Abstract — In this study, the effects of some parameters on the extraction of metals from copper converter slag by using a sulphuric acid/potassium dichromate lixiviant were investigated. The leaching
kinetic of copper from converter slag was also investigated. The results indicated that increasing the time
and temperature have positive effects on the extraction of metals. The best results were obtained by the
leaching of converter slag (-200 mesh, 10 g/L of slag/solution ratio) with a lixiviant containing 0.25 M
H2SO4 and 0.1 M K2Cr2O7 for 120 minutes at 70 °C. Under these conditions, 99.66% Cu extraction yield
was achieved. The kinetic evaluations showed that the extraction of Cu from the converter slag is well
represented by the shrinking core model controlled by diffusion through the slag matrix. The activation
energy of the leach process was calculated as 33.66 kJ/mol for the temperature range of 25 to 70 °C (298
to 343 K). Also, reaction order with respect to dichromate was determined as first order in the concentration range of 0.025 to 0.1 M at 25 °C (298 K).
Résumé — Dans cette étude, on a étudié l’effet de certains paramètres sur l’extraction de métaux à partir
des scories de convertisseur de cuivre en utilisant une solution de lixiviation d’acide sulfurique et de
bichromate de potassium. On a également étudié la cinétique de lixiviation du cuivre à partir des scories
de convertisseur. Les résultats ont indiqué que l’augmentation de la durée et de la température avait des
effets positifs sur l’extraction des métaux. On a obtenu les meilleurs résultats par la lixiviation des scories
de convertisseur (maille de –200, rapport de 10 g/L de scories/solution) avec une solution de lixiviation
contenant 0.25 M H2SO4 et 0.1 M K2Cr2O7 pendant 120 minutes à 70 °C. Sous ces conditions, on a
obtenu un rendement d’extraction de 99.66% de Cu. L’évaluation de la cinétique a montré que
l’extraction de cuivre à partir des scories de convertisseur était bien représentée par le modèle du noyau
rétrécissant contrôlé par diffusion à travers la matrice de scories. On a calculé l’énergie d’activation du
procédé de lessivage à 33.66 kJ/mol dans la gamme de température de 25 à 70 °C (298 à 343 K).
Également, on a déterminé l’ordre de réaction par rapport au bichromate comme étant de premier ordre
dans la gamme de concentration de 0.025 à 0.1M à 25 °C (298 K).
INTRODUCTION
The converter slag generated during the pyrometallurgical
copper production generally contains significant amounts of
some valuable metals such as copper, cobalt, nickel and zinc. In
order to recover most of the copper, returning the converter slag
to the smelter furnace is applied as a common practice. In this
case in addition to the operational problems encountered, the
volume and viscosity of the smelter slag are unnecessarily
increased and thus, the high copper loss occurred. As a result of
these problems, the converter slag needs to be discarded from
time to time. It has been shown that treating the converter slag
separately from the main pyrometallurgical cycle has the
advantages of low copper loss and high smelter capacity [1,2].
Alternatively, in the last few decades, there has been growing
interest in hydrometallurgical processes to recover the valuable
metals from copper smelting slags. In these studies, efforts are
mainly focussed on the leaching processes with or without some
pretreatments. Among the leaching materials searched, sulphuric
acid [3], cyanide [4,5], ammonia [6], ammonia-ammonium
carbonate mixture [7] and ferric chloride [8] can be mentioned.
In order to provide an enhanced solubilization, slags have been
subjected to sulphation-roasting techniques by using various
materials. As sulphation agents, sulphuric acid [9,10],
ammonium sulphate [9], ferrous sulphate [11], ferric sulphate
[12] and pyrite [13,14] have been used in roast-leach studies.
The other pretreatment technique prior to leaching is the
reduction-roasting process with carbonaceous materials. It has
been shown that various coals and furnace oil could be used as
reducing agents prior to ferric chloride leaching for the recovery
of metals from the copper converter slag [15]. In this context,
reduction with coal prior to acetic acid pressure leaching has also
CANADIAN METALLURGICAL QUARTERLY, VOL 45, NO 2
146
M. BOYRAZLI, H.S. ALTUNDOĞAN and F. TÜMEN
been studied [16]. On the other hand, these recovery methods
could not find any application facility on the industrial scale.
Copper can exist in the form of metal, sulphide and oxide
or their various combinations in the converter slag. While copper
in oxide form can readily be dissolved in leaching agents, copper
in metal and/or sulphide form necessitates oxidative conditions.
Pressure provided by using an oxygen containing gas is a
convenient route of creating oxidative leach conditions. It is well
known that the pressure leaching process using oxygen gas
provides a reduced extraction of iron which is considered as a
contaminant. In the acidic leaching processes applied under
oxygen pressure, Fe3+ ion is precipitated by hydrolyzing to its
insoluble compounds such as hematite, goethite and jarosite
[17]. Some researchers have reported that the mechanisms of
these processes are very complex [18,19]. It has been reported
that 90% of copper could be extracted with only 0.8% extraction
of iron from the converter slag by using pressure leaching [20].
Recently in some studies, dichromate compounds have been
considered as oxidizing agents for dissolving the sulphide
minerals. For this purpose, oxidation of pyrite [21] and
chalcopyrite [22] by using potassium dichromate and sulphuric
acid has been studied. Similarly, it has been shown that sodium
dichromate could be used as an oxidant to remove some copper
sulphide minerals from molibdenite concentrates [23].
The oxidative action of dichromate ion in acidic solutions
is based on its reduction according to [24].
Æ 2 Cr3+ + 7 H2O
(1)
Cr2O72- + 14 H+ + 6 e- ¨
The overall oxidation reaction for copper metal and/or sulphidic
copper compounds can be generalized as
Æ Cu2+
CuSx + (1/3+x) Cr2O72- + (14/3+6x) H+ ¨
(2)
23+
+ (2/3+2x) Cr + x SO4 + (7/3+3x) H2O
where x is a stoichiometric coefficient and its value depends on
the oxidation degree of copper (for example, x=0, 1/2 and 1
represent metallic copper, chalcosite and covellit, respectively).
In our earlier study, the effects of sulphuric acid and
potassium dichromate concentrations on the extraction of metals
from copper converter slag were investigated [25]. The results of
this study showed that the presence of dichromate has a large
influence on the extraction of metals. It was determined that the
copper extraction yields increased with dichromate concentration, while cobalt, zinc and iron extractions decreased
considerably probably due to some precipitation phenomena and
surface passivation effect caused by dichromate ions [25].
The aim of the present study is to investigate the effects of
the parameters such as temperature, time and slag/solution ratio
on the recovery of metals. Additionally, some kinetic evaluations
based on copper extraction were also made for the leaching
process.
EXPERIMENTAL
The converter slag sample used in this study was obtained from
Ergani Copper Plant, Maden, Elazig-Turkey. The slag sample
was crushed in a jaw crusher and ground in a ball mill and then
CANADIAN METALLURGICAL QUARTERLY, VOL 45, NO 2
sieved. The fraction of –74 mm (200 mesh) was used in all
experiments.
The potassium dichromate and sulphuric acid used in the
study were both reagent grade chemicals (K2Cr2O7, Riedel-De
Haan, 12255 and H2SO4, Riedel-De Haan, 7102). All other
chemicals used were of analytical reagent grade.
The slag samples were analyzed with an atomic
absorption spectrometer (Perkin-Elmer, 370) by using the
LiBO2 fusion-HNO3 dissolution route [26] for copper, cobalt,
iron and zinc. The sulphur, aluminum and silica contents of
converter slag were determined by gravimetric methods used
for sulphur and silica-based materials [27,28]. The converter
slag sample was subjected to X-ray diffraction analysis by
using a Shimadzu XRD-6000 diffractometer to identify its
mineralogical compositions. The mean particle diameter of the
slag sample was determined by a Malvern Inst MasterSizer X
particle size analyzer.
Batch leaching experiments were carried out by shaking
250 mL glass conical flasks containing a predetermined
amount of converter slag sample and 100 mL of solutions
having various concentrations of K2Cr2O7 and H2SO4. The
suspensions were shaken (400 min-1) by using a flask shaker
(Stuart Scientific SF1) equipped with a temperature controlled
water bath. At the end of the predetermined shaking period,
mixtures were filtered. The supernatants were analyzed by an
atomic absorption spechtrophotometry for Cu, Co, Fe and Zn.
Metal extractions were calculated from their individual concentrations in leach solutions and from slag composition.
In our recent study [25], the most suitable lixiviant
composition was determined as 0.1 M K2Cr2O7 and 0.25 M
H2SO4 keeping other parameters constant, i.e.,10 g/L of
solid/1iquid ratio, 25 °C temperature and 120 minutes of
contact time. In the present study, the effects of various
parameters such as temperature (25 to 70 °C), leaching time
(5 to 240 minutes) and solid/liquid ratio (5 to 400 g/L) on the
extraction of metals were studied by using the most suitable
lixiviant composition.
To make some kinetic analyses, the effect of temperature
(25 to 70 °C) and initial dichromate concentration (0.025 to
0.1 M), depending on time, were studied at fixed conditions of
0.25 M H2SO4 and 10 g/L slag/solution ratio.
The experiments were performed regularly in duplicate
and the mean values were considered. A group of experiments
were repeated a number of times to ascertain the
reproducibility of the results and the results were found to vary
within ±5 %.
RESULTS AND DISCUSSION
The chemical composition of the converter slag sample is
given in Table I. Fayalite (Fe2SiO4) and magnetite (Fe3O4)
phases were identified as major components by X-ray
analysis. Also, chalcosite (Cu2S) was determined as a minor
component. It was concluded that the converter slag had
significant copper, cobalt and zinc contents as well as a high
amount of iron.
RECOVERY OF METALS FROM COPPER CONVERTER SLAG BY LEACHING WITH K2Cr2O7-H2SO4
Table I – Chemical composition of the converter
slag sample
Constituents
%, w/w
Al
Co
Cu
Fe
S
Si
Zn
L.O.I. (100-1000 °C)
1.54
0.45
4.36
52.18
1.92
8.72
0.64
*-5.39
*Loss on ignition (negative value indicates that iron (II) compounds
convert to ferric oxide)
Particle size distribution analyses indicated that the
mean particle diameter of –200 mesh converter slag sample
used in the study was 25.3 mm.
147
increased Co, Zn and Fe extraction yields obtained at an
elevated leaching temperature may be attributed to decreasing
the adsorbed amounts of dichromate ions which are
responsible for passivation effects. As a result, it can be noted
that the amount of extracted iron, an important impurity,
increased significantly by increasing the temperature. While
the iron concentration was 0.167 g/L at 298 K, this value was
found to be 1.44 g/L at 70 °C. However, the extraction yield
of cobalt (about 42%) which is a valuable metal is not at a
satisfactory level at this leaching temperature.
Effect of Leaching Time
Figure 2 shows the effect of leaching time on the extraction of
metals from converter slag. As seen from this figure, amounts
of extracted metals increase with an increase in the contact
time. At the end of the contact period of 120 minutes, Cu, Co,
Fe and Zn extraction yields reached the values of 99.66,
42.09, 27.59 and 49.86%, respectively.
Effect of Leaching Temperature
The effect of temperature on the extraction of metals from
converter slag is shown in Figure 1. As seen, metal extraction
yields increase with temperature. The copper could be
completely extracted from slag at 70 °C. Also, extraction
yields of the other metals increased depending on
temperature. In our earlier study [25] in which the effect of
dichromate concentration on the extraction of metals from
converter slag with H2SO4 was investigated, the decrease in
the extraction of metals (i.e., Fe, Co and Zn) in oxide-silicate
matrices with an increase of dichromate concentration was
explained as a probable passivation caused by adsorption of
chromate ions on mineral surfaces. In the present study,
Fig. 1. Effect of temperature on the extraction of metals from converter slag
(slag/Solution ratio: 10g /L; K2Cr2O7 Conc.: 0.1 M; H2SO4 Conc.: 0.25 M;
Leaching time: 120 minutes).
Effect of Solid/Liquid Ratio
In the practical application of leaching processes, the
solid/liquid ratio is desired to be as high as possible in order
to obtain concentrated pregnant liquors. For that reason, the
effect of the slag/solution ratio on the extraction of metals was
investigated. Results of this study are shown in Figure 3. As
expected, extraction yields of all metals decrease with an
increase in the slag/solution ratio. However, although the
extraction yield of Cu is lower, higher slag/solution ratios can
be preferred to obtain concentrated pregnant solutions. For a
comparison, while the Cu extraction yields are 99.7 and
76.4% for 10 and 100 g/L slag/solution ratios, respectively;
Fig. 2. Effect of leaching time on the extraction of metals from converter
slag (slag/solution ratio: 10g/L; K2Cr2O7 Conc.: 0.1 M; H2SO4 Conc.: 0.25 M;
Temp.: 70 °C).
CANADIAN METALLURGICAL QUARTERLY, VOL 45, NO 2
148
M. BOYRAZLI, H.S. ALTUNDOĞAN and F. TÜMEN
Slag/Solution Ratio, gL-1
Fig. 3. Effect of slag/solution ratio on the extraction of metals from converter
slag (K2Cr2O7 Conc.: 0.1 M; H2SO4 Conc.: 0.25 M; Leaching time: 120 minutes; Temp.: 70 °C).
Fig. 4. Effect of temperature on the extraction rate of Cu from copper converter slag (slag/solution ratio: 10 g/L; K2Cr2O7 Conc.: 0.1 M; H2SO4 Conc.:
0.25 M).
Cu concentrations of leach solutions increase about eight
times (from 0.43 to 3.33 g/L) for corresponding slag/solution
values. The copper loss in the case of high slag/solution ratio,
however, could be reduced by applying a countercurrent
leaching system in practice.
Further, the amount of iron per unit amount of copper
passed into pregnant solution is another important issue. This
ratio (kg Fe/kg Cu) was found to significantly decrease with
increasing slag/solution values. For example, while the Fe/Cu
ratio is about 3.2 for 10 g/L pulp density, this value is about
1 for that of 100 g/L.
Leaching Kinetics
Cu leaching from converter slags by using lixiviant
containing K2Cr2O7 and H2SO4 is a complex heterogenous
process. In addition to revealing the effect of leaching
parameters, it will also be useful to have kinetic information.
Kinetic analyses were made based on the data of the effect of
temperature and K2Cr2O7 concentration on the Cu extraction
depending on the leaching time. The effects of temperature
and K2Cr2O7 concentration on the Cu extraction rate are
shown in Figures 4 and 5, respectively.
The Cu extraction yields obtained depending on time
and temperature (Figure 4) were applied to various heterogeneous kinetic models [29]. The experimental data were well
fitted to the relationship between time and conversion for the
shrinking core model which assumes the diffusion through the
ash (slag matrix) is a rate limiting step (Equation 3).
1-3 (1-X)2/3 + 2 (1-X) = k t
(3)
where X is the extraction of Cu, t is time (min) and k is the
apparent rate constant (min-1). From Equation 3, the
CANADIAN METALLURGICAL QUARTERLY, VOL 45, NO 2
Fig. 5. Effect of K2Cr2O7 concentration on the extraction rate of Cu from
copper converter slag (slag/solution ratio: 10 g/L; 0.1 M; H2SO4 Conc.:
0.25 M; Temp.: 25 °C).
variation of 1-3 (1-X)2/3 + 2 (1-X) with time were plotted as
shown in Figure 6. The apparent rate constant, k, was
obtained from the slopes of the lines in the figure.
Calculated apparent rate constants and correlation coefficients (R2) for various temperatures are given in Table II.
The Arrhenius equation was used to calculate activation
energy of leach process by plotting lnk versus 1/T (Figure
7). The activation energy for Cu extraction from the copper
converter slag was calculated as 33.66 kJ/mol. This value
may be considered as high for a process controlled by
diffusion. However, for a mechanism governed by diffusion
RECOVERY OF METALS FROM COPPER CONVERTER SLAG BY LEACHING WITH K2Cr2O7-H2SO4
149
Table II – Correlation coefficients and apparent rate
constants for extraction of Cu from copper
converter slag at various temperatures
Temperature, °C
k¥103, min-1
R2
25
40
55
70
0.923
1.220
4.631
7.129
0.9948
0.9915
0.9905
0.9952
Table III – Correlation coefficients and apparent rate
constants for extraction of Cu from copper
converter slag at various initial concentrations of
potassium dichromate
Fig. 6. A plot of 1-3 (1-X)2/3+2 (1-X) versus time for various temperatures
during Cu extraction from converter slag by potassium dichromate leaching
(slag/solution ratio: 10g/L; K2Cr2O7 Conc.: 0.1 M; H2SO4 Conc.: 0.25 M).
Fig. 7. Arrhenius plot for the Cu extraction from converter slag by potassium
dichromate leaching.
control, activation energies reported in the literature for
leaching of copper sulphide minerals by various lixiviants
are in the range of 33.5 to 67 kJ/mol [23,30,31].
The data obtained from experiments carried out for
various dichromate concentrations depending on time (Figure 5)
were also applied to this leaching model. Calculated apparent
rate constants for various initial dichromate concentrations
are given in Table III. Reaction rate order with respect to
dichromate concentration can be determined from the slope of
the line obtained for lnk versus lnC (Figure 8). This figure
shows that Cu leaching from converter slag is a first order
reaction with respect to the dichromate concentration in the
interval of 0.025 to 0.1 M.
K2Cr2O7 Conc., M
k¥104, min-1
R2
0.025
0.050
0.100
2.091
2.711
9.229
0.8649
0.9267
0.9948
Fig. 8.
Plot of Cu leaching rate versus initial K2Cr2O7 concentration
(slag/solution ratio: 10 g/L; Temp.: 25 °C; H2SO4 Conc.: 0.25 M).
CONCLUSIONS
From the results of this study, the following conclusions can
be drawn:
1. Extraction yields of all metals increased with increasing
temperature and time and decreasing slag/solution ratio.
Also, Fe/Cu ratio (kg/kg) in solution, an important
parameter for practical application of such processes,
decreased by increasing the slag/solution ratio. On the
other hand, Cu concentrations in the pregnant liquor were
found to be higher for increased slag/solution ratios.
Thus, in spite of the lower extraction yields obtained for
Cu, increased slag/solution ratios may be preferred.
CANADIAN METALLURGICAL QUARTERLY, VOL 45, NO 2
150
M. BOYRAZLI, H.S. ALTUNDOĞAN and F. TÜMEN
The kinetics of Cu extraction from converter slag by
using dichromate can be described by means of the
diffusion controlled shrinking core model. Activation
energy for this process for the temperature range of 25 to
70 °C (298 to 343 K) was calculated as 33.66 kJ/mol.
3. Cu leaching rates with respect to dichromate concentration, in the range of 0.025 to 0.10 M, was determined
as first order.
A proposed flow sheet for metal recovery from converter
slag by this process is illustrated in Figure 9. As seen, following
the liquid-solid separation after the leaching, solubilized
copper, cobalt, nickel and zinc can be separated from the
pregnant solution containing dichromate by using a sulfide
precipitation. For this reason, sodium sulfide can be used as a
precipitation agent. Sulfide precipitation may require a pH
adjustment to increase the sulfide formation yield. After the
precipitation, the metal sulfide concentrate obtained can be
utilized pyrometallurgically. On the other hand, the liquid
fraction obtained from the precipitation stage can be used to
obtain regenerated lixiviant solution by adding sulfuric acid
and dichromate. Also, in this stage, reduced chromium (Cr3+)
during the leaching may be reoxidized to dichromate by using
some oxidation agents such as MnO2 or H2O2.
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This study was supported by the Research Foundation of Fırat
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